Effect of various staining beverages on the color stability of CAD/CAM PMMA denture teeth: An in vitro study

Abstract Objective This study aimed to compare the color change of computer‐aided design (CAD)/computer‐aided manufacturing (CAM) polymethyl methacrylate (PMMA) denture teeth and conventional acrylic teeth after immersion in three staining beverages (coffee, red tea, and cola) for a day, 7 days, and 30 days. Materials and Methods Group 1: Conventional acrylic teeth (n = 32). Group 2: Milled CAD/CAM teeth out of PMMA disc (n = 32). The specimens of each material were further divided into four subgroups: (1) Control group, distilled water (n = 16). (2) Red tea solution (n = 16). (3) Coffee solution (n = 16). (4) Cola (n = 16). The color change (∆E) was assessed using a spectrophotometer at four time points: at the baseline (t0), on the 1st day (t1), on the 7th day (t2), and the 30th day (t3) of immersion. Kolmogorov–Smirnov test was applied, followed by performing independent samples t test, one‐way analysis of variance and post‐hoc Tukey tests to compare the color change values at different time points. Results The mean score of NBS values of the coffee solution indicates perceivable color change at the end of the 30th day in the conventional acrylic teeth group. It was 0.843 ± 0.395 at t1, then increased to 1.017 ± 0.477 at t2 and to 2.259 ± 1.059 at t3. There is a statistically significant difference (p < 0.05) in color change values between both tooth types at the end of the 30th day of immersion in red tea solution and a statistically significant difference at the end of the 7th day (p < 0.05) and the 30th day (p < 0.05) of immersion in coffee solution. Conclusions CAD/CAM PMMA teeth are more color stable than conventional acrylic teeth after 30 days of immersion in coffee and red tea solution.


| INTRODUCTION
Although the proportion of complete edentulous patients has decreased, the percentage of partial edentulism is increasing due to the increased life expectancy (Al-Rafee, 2020).According to the World Health Organization, the number of people aged 60 years or over has been increasing rapidly in recent years (Al-Rafee, 2020; Rudnicka et al., 2020).Fixed dental prostheses (FDPs) and implantsupported fixed prostheses are the most appropriate treatment options for partially edentulous patients.However, FDPs are contraindicated when there are unfavorable supporting abutments and multiple missing teeth (Alenezi & Aloqayli, 2023).In addition, dental implants are a high-cost and time-consuming option, especially when the implant site requires additional therapies (Alenezi & Aloqayli, 2023).In contrast, removable partial dentures (RPDs) are time and cost savings, provide aesthetic support, and are viable rehabilitation options for long edentulous spans.Therefore, the use of RPDs is still necessary, and it will be growing even more in the future (Mousa et al., 2021).
Conventional acrylic resin artificial teeth are widely used in prosthetic rehabilitation due to their advantages, such as biocompatibility, high flexural strength, fracture resistance, and bond strength to acrylic resin denture bases.In addition, they have good elastic properties due to their ability to absorb masticatory forces.
However, they present aesthetic problems since they have low abrasion wear resistance and high staining susceptibility (Muhammad et al., 2022).Polymethyl methacrylate (PMMA) is a synthetic polymer free of inorganic fillers, which was introduced in 1936, and it is widely used for various dental applications owing to its unique properties.
PMMA has several advantages, such as good physical and mechanical properties, low density, low cost, and biocompatibility.However, adding fibers or nanoparticles can compromise the aesthetic properties of PMMA, such as color and translucency.In addition, PMMA presents inferior wear resistance, which leads to staining after some time (Zafar, 2020).Computer-aided design (CAD)/computeraided manufacturing (CAM) technology reduces chairside time, improves the aesthetic and mechanical properties of dental materials, and enhances the quality of different prosthetic restorations.
However, the CAD/CAM technique is highly expensive and requires trained staff (Alghazzawi, 2016;Islam et al., 2023).CAD/CAM PMMA is superior to conventionally heat-cured PMMA in terms of mechanical features such as impact strength, flexural strength, hardness, and flexural modulus.Regarding biocompatibility, CAD/ CAM PMMA is biocompatible with a minimal leaching residual monomer.In addition, CAD/CAM PMMA was superior to conventional heat-cured PMMA surface hardness, surface wettability, surface roughness, and hydrophobicity, which affect acrylic discoloration (Al-Dwairi et al., 2020;Zafar, 2020).Therefore, this study aimed to evaluate and compare the color change of CAD/CAM PMMA denture teeth and conventional acrylic teeth after immersion in three staining beverages (coffee, red tea, and cola) for a day, 7 days, and 30 days.The null hypothesis was that no statistically significant difference would be detected in the color stability between conventional heat-cured PMMA and CAD/CAM PMMA after immersion in various staining beverages at different times.

| Sample size and grouping
This was an in-vitro study.The sample size was calculated using the G.Power 3.1.9software (G*Power 3.1.9,Heinrich Hein Universität  3) Coffee (Café Shami, Al-Shami-Cafe Co.) (n = 16).( 4) Cola (Pepsi, PepsiCo, Inc.) (n = 16).The primary endpoint was the color change of CAD/CAM PMMA denture teeth and conventional acrylic teeth after immersion in the study medium for a day, 7 days, and 30 days.

| Immersion solutions and specimen preparation
To prepare the tea solution, two tea bags (2 × 2 g) were immersed in boiling water, and the final volume of the tea solution was 300 mL following the tea waste removal.The coffee solution was prepared by solving 2 g of coffee in 100 mL of boiling water, and both solutions were filtered and left until they cooled to 37°C.Moreover, 100 mL of cola and distilled water were poured as received (Al-Shami et al., 2023).All solutions were poured into separate dishes, stored in an incubator (Classic Incubators, LEEC Ltd.) at 37°C, and stirred three times a day to avoid sedimentation.Every day, the immersing solutions were replaced to prevent contamination (Al-Shami et al., 2023).
The specimens of the second group were milled out of PMMA disc using a CAD/CAM dental milling machine (DWX-52DCi, Ronald DGA Co.) (Figure 1) (Soeda et al., 2022), and the milled teeth were corresponding in size, shape, and color to the conventional acrylic teeth.The teeth were immersed in the study solution for a day, 7 days, and 30 days (Aydın et al., 2020).Each medium contains eight specimens of each group with no contact with each other (Aydın et al., 2020).

| Statistical analysis
IBM SPSS software version 24 (IBM SPSS Statistics ® version 24, IBM Corp.) was used to perform statistical analysis.Kolmogorov-Smirnov test was applied to check the normality of data, followed by performing an independent samples t test to compare the color change values of both tooth types in the study medium at different time points.Oneway analysis of variance (ANOVA) and post-hoc Tukey tests were performed to compare the cumulative effect of the study medium at different time points in each group.Descriptive statistics were presented as mean, standard deviation, standard error, minimum, and maximum.The level of significance was set at 0.05 (p < 0.05).

| RESULTS
Descriptive statistics of the color change values of study groups at different time points are listed in Table 1.The mean score of NBS values of the coffee solution indicates perceivable color change at the end of the 30th day (2.259 ± 1.059) in the conventional acrylic teeth group.Regarding red tea and cola, the mean score of NBS values refers to a slight color change for both tooth types at different time points.In the cola immersion group, the mean score of NBS value for CAD/CAM PMMA teeth was 0.655 ± 0.307 at t 1 , then increased to 0.870 ± 0.408 at t 2 and to 0.936 ± 0.439 at t 3 .Similarly, the mean score of NBS value for conventional acrylic teeth was 0.645 ± 0.302 at t 1 , then increased to 0.870 ± 0.408 at t 2 and to 0.934 ± 0.438 at t 3 .Therefore, the level of discoloration of CAD/ CAM PMMA teeth and conventional acrylic teeth increased with time after immersion in cola beverage.In addition, regarding conventional acrylic teeth, the mean score of NBS value was 0.714 ± 0.335 at t 1 , then increased to 0.952 ± 0.446 at t 2 and to 1.529 ± 0.717 at t 3 after immersion in red tea solution.Similarly, the mean score of NBS value was 0.843 ± 0.395 at t 1 , then increased to 1.017 ± 0.477 at t 2 and to 2.259 ± 1.059 at t 3 after immersion in coffee solution.Hence, the level of discoloration of conventional acrylic teeth increased with time after immersion in red tea and coffee solution.Regarding the distilled water group, the mean scores of NBS values indicate an extremely slight color change.
The results of the independent samples t test showed a statistically significant difference (p < 0.05) in color change values between both tooth types at the end of the 30th day of immersion in red tea solution.However, no statistically significant difference was noted at the end of the 1st day (p = 0.972) and the 7th day (p = 1.000) of immersion (Table 2 and Figure 2).In the coffee solution immersion group, there was a statistically significant difference between the two types of teeth at the end of the 7th day (p < 0.05) and the 30th day (p < 0.05).However, no statistically significant difference was noted at the end of the 1st day (p = 0.897) (Table 3 and Figure 3).No statistically significant difference was noted between the two types of teeth at t 1 (p = 0.896), t 2 (p = 1.000) and t 3 (p = 0.986) in the cola beverage immersion group (Table 4).Similarly, no significant difference was detected at t 1 (p = 0.343), t 2 (p = 0.962), and t 3 (p = 0.971) in the distilled water immersion group (Table 5).
Results of a one-way ANOVA post-hoc Tukey test for comparison of color change values after teeth immersion in the study medium are listed in Tables 6 and 7. Red tea solution had the highest cumulative effect after 30 days of immersion compared with other mediums (p < 0.05) in the CAD/CAM PMMA teeth group (Table 6).
Regarding the conventional acrylic teeth group, all staining beverages had a cumulative effect after 30 days of immersion compared with distilled water (p < 0.05) (Table 7).

| DISCUSSION
Acrylic removable appliances are still the best treatment option for complete or partial edentulous patients.Therefore, the materials of these dental appliances must have high mechanical and physical properties (Mousa et al., 2021).As the patient's interest in cosmetic aspects has increased, the aesthetic features became an essential requirement for successful prosthetic treatment.Color stability of removable appliances is an important clinical aspect, as the appearance and color of teeth must match the surrounding dental tissue and adjacent teeth, and this is mainly related to the selection of biomaterials that have long-term color stability.Therefore, the color  stability of acrylic teeth and their resistance to staining have become a critical demand (Donovan et al., 2001).With developments in dental materials, including acrylic, many techniques and types of acrylic teeth have emerged, which have increased in popularity due to the noticeable improvement in their properties.All these factors led to the introduction of new types of artificial teeth to suit these new requirements, such as milled CAD/CAM teeth out of PMMA discs (Al-Dwairi et al., 2020;Zafar, 2020).Hence, the need arose to evaluate the resistance of these teeth to staining when exposed to various beverages.Therefore, this study aimed to compare the color change of CAD/CAM PMMA denture teeth and conventional acrylic teeth after immersion in three staining beverages (coffee, red tea, and cola) for a day, 7 days, and 30 days.To the best of the authors' knowledge, no study has ever evaluated the discoloration of CAD/CAM PMMA teeth compared with conventional acrylic teeth.
In this study, color measurement was performed according to the CIEL*a*b* (CIELAB) color scale using a spectrophotometer.
According to Paul et al. (2002)., spectrophotometric shade assessment is more reproducible and accurate than human shade analysis.et al., 2023;Takhtdar et al., 2023) which considered an optimal period to obtain a cumulative effect.In addition, according to Aydin et al. ( 2020), 30 days of exposure to certain beverages would reproduce 2.5 years of regular consumption, and a day of exposure would reproduce 30 days of consumption.
The result of the current study showed that the mean score of NBS values of the coffee solution indicates perceivable color change at the end of the 30th day in the conventional acrylic teeth group.This result could be attributed to the fact that discoloration results from the presence of yellow dyes and acids, including tannic acid.In addition, acidification of coffee produces high-molecular-weight, brown-colored nitrogenous compounds called melanoidins, which are primarily responsible for discoloration (Nunes & Coimbra, 2007).This finding is in agreement Regarding the coffee and red tea immersion group, the level of discoloration of conventional acrylic teeth increased with time after immersion in coffee and red tea solution compared with CAD/CAM PMMA teeth.The higher staining in conventional acrylic teeth is due to the difference in the structure of each type of tooth, as the molecular structure of acrylic teeth tends to absorb water, which causes pigmentation.Absorbed water molecules separate the polymer chains, which leads to the penetration of the staining beverage (Neppelenbroek et al., 2015).
In addition, according to Arslan et al. (2018) | 9 color stability.As previously conducted for other dental materials, future studies are needed to test other important characteristics such as hardness (Colombo et al., 2020), fatigue (Rodríguez-Ivich et al., 2022), and flexural strength (Cacciafesta et al., 2008) to complete the knowledge about CAD/CAM PMMA denture teeth.
This study has some limitations.The use of specific types of beverages and continuous immersion does not match the oral environment, as laboratory study conditions cannot accurately simulate the oral environment.The coloration is affected by chemical and mechanical changes in the oral cavity, such as the composition of saliva, differences in foods and drinks, mastication efforts, and temperature fluctuations.In addition, abrasion and corrosion can increase discoloration, which also relates to oral health, dietary habits, and the care of the removable denture.

F
I G U R E 1 Milled CAD/CAM teeth out of PMMA disc.T A B L E 1 Descriptive statistics of the color change values of both tooth types in the study medium at different time points.
histogram of color change values of both tooth types in red tea solution at different time points.T A B L E 3 Results of independent samples t test for comparison of color change values of both tooth types in coffee solution at different time points.
Coffee, red tea, and cola were assessed in the current study as they are the most consumed staining beverages in the daily diet(Al-Shami et al., 2023;Aydın et al., 2020), and distilled water was selected as the control group.According to Al-Qarni et al. (2020) and Dayan et al. (2019), distilled water is a preferred control group when comparing staining beverages.The samples were immersed in the medium for a day, 7 days, and 30 days.The maximum duration of F I G U R E 3 A histogram of color change values of both tooth types in coffee solution at different time points.T A B L E 4 Results of independent samples t test for comparison of color change values of both tooth types in cola beverage at different time points.
with the one reported byMutlu-Sagesen et al. (2001), which suggested that the most staining beverage for porcelain, conventional, and reinforced acrylic teeth is filtered coffee after 30 days of immersion.Likewise, according toGregorius et al. (2012), coffee beverage causes the largest color shift in hue, value, and chroma for high-strength acrylic denture teeth after 7 days of immersion.No statistically significant difference was noted between the two types of teeth at different time points in the cola beverage immersion group, and the mean score of NBS values refers to a slight color change.This result is consistent withMousavi et al. (2016) finding, which suggested that cola causes a slight color change in three different brands of acrylic teeth after 6 weeks of immersion.In addition,Koksal and Dikbas (2008) concluded that according to NBS values, cola causes a slight color change in acrylic and porcelain denture teeth after 30 days of immersion.Furthermore, Aydin et al. (2020) study found that cola causes staining below the clinically perceptible level in different CAD/CAM resin blocks at the end of the 30th day.
Results of independent samples t test for comparison of color change values of both tooth types in distilled water at different time points.
Results of a one-way ANOVA post-hoc Tukey test for comparison of the cumulative effect of the study medium at different time points in the CAD/CAM PMMA teeth group.
(Al-Dwairi et al., 2020;Zafar, 2020)pores, and greater color stability(Al-Dwairi et al., 2020;Zafar, 2020).However, this result is in contrast with Al-Qarni et al. (2020) finding, which suggested that CAD/CAM PMMA teeth have a similar color change to conventional acrylic teeth when immersed in different staining beverages.The present report evaluated staining and T A B L E 6